A number of inorganic materials have been shown to possess antimicrobial activity. They include metal ions such as silver, copper, zinc, mercury, tin, lead, bismuth, cadmium, chromium and thallium ions. It is theorized that these antimicrobial metal ions exert their effects by disrupting respiration and electron transport systems upon absorption into bacterial or fungal cells. Antimicrobial metal ions of silver, copper, zinc, and gold, in particular, are considered safe for in vivo use. Antimicrobial silver ions are particularly useful for in vivo uses due to the fact that they have the highest ratio of antimicrobial efficacy to human toxicity.
Antimicrobial zeolites can be prepared by replacing all or part of the ion-exchangeable ions in zeolite with antimicrobial metal ions, as described in U.S. Pat. Nos. 4,911,898; 4,911,899; 4,938,955; 4,906,464; and 4,775,585.
Zirconium compounds, such as zirconium phosphates, have also been modified to provide antimicrobial characteristics, as described in U.S. Pat. Nos. 4,025,608 and 4,059,679. J. Antibact. Antifung. Agents Vol. 22, No. 10, pp. 595-601, 1994 and the references therein describe the antimicrobial characteristics of zirconium phosphate ceramics.
Antimicrobial water soluble glasses have been used and are described in U.S. Pat. No. 5,470,585.
Antimicrobial hydroxyapatite powders have been prepared and are described in U.S. Pat. Nos. 5,009,898 and 5,268,174.
U.S. Pat. No. 4,775,585 discloses incorporating metal-zeolite into a polymer to obtain a polymer with bactericidal activity. U.S. Pat. No. 4,923,450 discloses incorporating zeolite in bulk materials for production of medical tubes.
U.S. Pat. No. 6,436,422 discloses an antimicrobial-coated substrate comprising an antimicrobial coating composition coated on a substrate. The antimicrobial coating composition comprises a hydrophilic polymer having antimicrobial ceramic particles dispersed therein.
U.S. Pat. No. 5,238,749 describes a two-layer antimicrobial coating with a top layer using a thermoplastic material selected from the group consisting of nylon 6, nylon 6-6, nylon 11, polyvinylidene fluoride polymer and the family of polyethylene thermoplastic resins, with nylon 11 being preferred. The antimicrobial agent is 5-chloro-2-(2,4-dichlorophenoxy)phenol or polyhexamethylene biguanide hydrochloride. The selection of the organic antimicrobial agent is based on its ability to migrate through the polymer due to the antimicrobial agent's low vapor pressure. However, the use organic antimicrobial agents with low enough vapor pressures oftentimes results in poor surface appearance and limited life due to the ease with which the antimicrobial agent passes through the polymer and blooms to the surface. The tendency is for these organic antimicrobial materials to bloom to the surface continually until the supply or concentration of material is exhausted.
The use of inorganic antimicrobial agents, particularly those whose activity is based on antimicrobial metal ions, can sometimes overcome these problems. However, these antimicrobial agents rely upon moisture or another solvent to dissolve/dissociate and/or transport the active antimicrobial agent. With the ion-exchange type inorganic antimicrobial agents, moisture is needed to carry in the ions to be exchanged and carry out the antimicrobial ions. In polymers such as nylon 11, transport of antimicrobial metal ions can be poor, if existent at all, due the non-hydrophilic nature of the polymer. Specifically, nylon 11 typically manifests a moisture absorption capability of only about 0.2%. This low level of moisture is, at best, marginally sufficient to provide adequate ion exchange and transport through the polymer matrix in which the antimicrobial agent is dispersed. Consequently, such systems have limited antimicrobial efficacy due to poor transport of the metal ions.
One of the problems in the prior art is that coating systems often require a compromise amongst several desirable properties. Formulating a coating with a hydrophilic polymer enables the use of antimicrobial metal ions and gives the desired initial boost of antimicrobial effectiveness. However, these coatings are typically easily abraded in any erosive environment and do not give lasting protection. By choosing non-hydrophilic polymers as the coating matrix, the wear properties can be improved due to the stronger physical performance characteristics and properties, but there are problems with obtaining good antimicrobial activity. Oftentimes these polymer materials skin over the antimicrobial agent, preventing the direct exposure of the antimicrobial agent to the coating surface. This results in the unavailability, for the most part, of that quantity of the antimicrobial agent that lies beneath the surface of the coating into which it is incorporated until the antimicrobial agent is exposed by erosion. Again, due to the low moisture absorption of the non-hydrophilic polymers, migration of the antimicrobial ions can be poor, if existent at all. Thus, the entombed antimicrobial agent is without utility or efficacy. Where the matrix does not completely entomb the antimicrobial particles, oftentimes the use of a larger quantity of antimicrobial agent is required so as to provide a higher concentration at the surface. This, however, is more costly and often imparts deleterious properties. These problems can be exacerbated by surfactants and leveling agents commonly used in coating systems and designed to form a skin at the surface of the coating to control surface finish. This same skin can also form over the antimicrobial agent.
There remains a need to provide an antimicrobial coating in a form that is suitable to impart antimicrobial properties without the accompanying problems of the prior art. More specifically, there remains a need to provide an antimicrobial coating that provides excellent antimicrobial activity upon application of the coating as well as good long-term antimicrobial activity and durability.